A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Another lithographic system is an interferometric lithographic system where there is no patterning device, but rather a light beam is split into two beams, and the two beams are caused to interfere at a target portion of substrate through the use of a reflection system. The interference causes lines to be formed on at the target portion of the substrate.
During lithographic operation, different processing steps may require different layers to be sequentially formed on the substrate. Accordingly, it may be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. Lithographic apparatus may use an alignment system for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask.
Alignment systems typically have their own illumination system. The signal detected from the illuminated alignment marks may be dependent on how well the wavelengths of the illumination system are matched to the physical or optical characteristics of the alignment marks, or physical or optical characteristics of materials in contact with or adjacent to the alignment marks. The aforementioned characteristics may vary depending on the processing steps used. Alignment systems may offer a narrow band radiation beam having a set of discrete, relatively narrow passbands in order to maximize the quality and intensity of alignment mark signals detected by the alignment system.
Alignment systems are usually configured for optimal performance at a desired center wavelength (CWL) value of the one or more narrow passbands of the narrow band radiation beam filtered from a broad band radiation beam and output by the illumination system. However, the actual CWL values of the one or more narrow passbands incident on the alignment marks may be different from the desired CWL values. The shift of the actual CWL values from the desired CWL values may be due to, for example, mechanical disturbances to the illumination system, degradation of illumination system components with time, replacement of one or more components of the illumination system, and/or thermal effects from the operation of the lithographic apparatus. This shift in the CWL values may result in inaccurate alignment measurements of the alignment system.